Pseudopotential total-energy study of the transition from rhombohedral graphite to diamond

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Abstract
The path maintaining rhombohedral symmetry in the transition from graphite to diamond which minimizes the energy at each value of the bond length between layers is determined. The energy barrier for this path is found to be 0.33 eV. The total energy of the solid is calculated using local-density-functional theory with ab initio pseudopotentials. Results are presented for the charge density and density of states along the transition path. In contrast to recent extended-Hückel-theory results, throughout most of the transition the structure is found to remain semimetallic or semiconducting. A final rapid opening of the gap to the insulating diamond phase develops as the interlayer carbon-carbon bonds form. The behavior of rhombohedral graphite under conditions of isotropic pressure is also examined. We predict that rhombohedral graphite will transform to diamond, without thermal or catalytic activation, at an isotropic pressure of 80 GPa if it maintains its rhombohedral symmetry. Our analysis moreover suggests that, in general, cross linking of hexagonal-ring carbon compounds leading to local tetrahedral coordination should be favored when the interlayer distance between hexagonal rings is between 2.1 and 2.3 Å.

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